1. Field of the Invention
The present invention relates to the field of extrusion and, in particular, to those extrusion processes wherein the object being extruded, e.g. a billet of metal or other extrudable material, is forced through a die by mechanical or hydrostatic means. In particular, the invention pertains to those extrusion processes where successive extrusions are accomplished on the original billet in a step-wise fashion. More particularly, the invention pertains to multiple die arrangements in order to provide greater extrusion reductions on a given extrusion press or apparatus wherein a solid or hollow billet undergoes multiple reductions without increasing the extrusion pressure.
2. Description of the Prior Art
Extrusion processes have been used for many years for producing semi-finished shapes in metals such as bars, wire, tubing, and complicated finished shapes such as H's, angles, and the like. Conventional extrusion processes are employed for both hot and cold extrusion, e.g. where the billet undergoing extrusion is either raised to an elevated temperature or is extruded at ambient, the former being more common for metals and the latter for plastics. The use of an elevated temperature will generally depend upon the material being extruded, the size of the initial billet, and the size and shape being extruded. Both hot and cold processes also encompass the use of lubricants and other aids to minimize the friction between the die and the material being extruded. Conventional extrusion processes are illustrated in U.S. Pat. Nos. 2,123,416 and 2,135,193. Extrusion dies used in such processes, and in particular in a multiple die set are illustrated in U.S. Pat. No. 3,553,996.
More recently, the hydrostatic extrusion technique has become widely adopted for use in extruding materials heretofore difficult to extrude by techniques illustrated by the above patents or materials that, because of their propensity to oxidize at elevated temperature or materials that require closer dimensional control, are better suited to cold extrusion. Generally, cold extrusion of such materials by conventional techniques requires equipment capable of very high extrusion pressures. In the hydrostatic technique, a fluid raised to an elevated pressure forces the billet through the die to achieve the final shape. An excellent discussion of the history of hydrostatic extrusion is contained in the specification of U.S. Pat. No. 3,491,565. Hydrostatic extrusion processes are illustrated in U.S. Pat. Nos. 3,126,096; 3,343,388; 3,677,049; and 3,893,320. One type of hydrostatic extrusion die is illustrated in U.S. Pat. No. 3,583,204.
In addition, materials can be and have been formed into elongated shapes by drawing through a die. Such processes are illustrated in U.S. Pat. No. 3,740,990.
It is well known in the art that the maximum allowable reduction for a billet undergoing an extrusion process is limited by the extrusion pressure applied to the billet as it enters the extrusion die, the billet material flow stress, and the die friction. In both hot and cold extrusion, whether conventional or hydrostatic, the limits of allowable stresses in the components of the extrusion apparatus (extrusion chamber, ram, and dies) restrict the maximum extrusion pressure that can be produced with a given apparatus. Thus, regardless of the design of the extrusion apparatus, the maximum extrusion pressure and resultant reductions accomplished by that pressure are limited by the materials of construction of the apparatus.
The reduction limit means that, for many extruded products, the initial billet must be limited in cross-sectional area, otherwise the reduction will have to be accomplished in successive steps. This size limitation is most severe in conventional and hydrostatic extrusion processes that are carried out at ambient temperature (so-called cold extrusion processes) because of the high flow stress and work hardening of the material being extruded. The advantage of large extrusion reductions, associated with hot extrusions, must be sacrificed if the improvements in tolerances and properties resulting from cold extrusion are desired or necessary. As pointed out above, cold extrusion may be the only process available if the material is one that oxidizes readily, or suffers some other form of degradation, at elevated temperature.
It is well known that hydrostatic extrusion processes are advantageous in that: (1) high pressures can be applied to the billet for greater reduction ratios; (2) there is generally a small die cone angle; (3) the extruded product can be made to close dimensional tolerances; (4) conditions of good lubrication exist; and (5) there is less die wear. However, hydrostatic extrusion may not be available for use with some products because the extruded product volume (primarily determined by the long length required) necssitates an initial billet, which is too large in diameter to be extruded in one step. When hydrostatically extruding certain materials, it becomes necessary to extrude the material into a pressurized container to increase the hydrostatic stress state during the forming operation. This process requires a pressurized container of sufficient size to accept the entire extrusion product which limits the pressure differential across the extrusion die, thus further restricting the reduction ratio between the initial billet and the extrusion product.
One problem often associated with hydrostatic extrusion is the unstable, so-called stick-slip extrusion action, which is usually minimized or eliminated by some form of mechanical action such as pushing on the billet or pulling on the extrusion. This unstable extrusion action is essentially an unsolved problem when large reductions are attempted on cold or elevated temperature (below crystalline melting temperature) polymeric materials using the hydrostatic extrusion process.